The present invention relates to gas discharge devices. In particular, the present invention relates to gas discharge light sources having vacuum ultraviolet (“VUV”) radiation-absorbing phosphors.
Light generation in mercury low-pressure gas discharge lamps is based on the principle that during the gas discharge, a plasma develops in the mercury-containing gas filling within a lamp and emits electromagnetic (“EM”) radiation predominantly in the ultraviolet (“UV”) wavelength range. The UV radiation emitted by a low-pressure mercury discharge concentrates at wavelengths of about 254 nm and about 185 nm. A phosphor, which is typically coated on the lamp housing, is employed to convert the UV radiation into visible light.
A drawback of known mercury low-pressure gas discharge lamps resides in that the action of UV radiation having very short wavelengths on the phosphor coating, the recombination of mercury ions and electrons on the phosphor surface, or the incidence of excited mercury atoms and electrons on the phosphor layer causes the emissivity of the phosphors to decrease in the course of time. This emissivity loss results in a reduction of the electro-optical efficiency in the course of the service life of the lamp. This degradation is particularly substantial under the influence of VUV (vacuum UV) radiation having a wavelength below 200 nm, and manifests itself as a shift of the color of light.
Therefore, effort has been devoted to finding means to reduce the damaging effect of VUV radiation on the phosphors of mercury discharge lamps. For example, U.S. Pat. No. 5,624,602 discloses the use of a mixture of phosphor particles and particles of aluminum oxide having a particle size of about 20 nm for a coating on the inner surface of the lamp housing. The aluminum oxide particles absorb the VUV radiation, but are transparent with respect to UV radiation having wavelength of 254 nm. As can be appreciated, this method would still allow some VUV radiation to be incident on the phosphor particles. In addition, aluminum oxide does not emit any light, thus, does not help to increase the efficiency of the lamp. U.S. patent application Ser. No. 2003/0011310 A1 discloses a two-layer phosphor coating. The first layer exposed to the mercury discharge is a very dense layer of a phosphor that is capable of absorbing VUV radiation and emitting in the wavelength range of 230 to 280 nm. The second layer is disposed between the first layer and the inner surface of the lamp housing and includes phosphors that absorb the longer-wavelength UV radiation and emit visible light. This method would add some manufacturing cost because of the need to form an additional very dense top layer (the first layer) to protect the underlying phosphor layer.
Therefore, it is desirable to continue to search for other means to reduce the damaging effect of the VUV radiation on the phosphors used in mercury discharge lamps to maintain the luminous output of these devices. It is also very desirable to achieve this goal and to improve the electro-optical efficiency of these devices at the same time.